Add Mintlify version of the docs

[k-finken]

This adds a Mintlify version of the GPT Researcher docs in a new mintlify/ directory. The existing Docusaurus site in docs/ is untouched, so nothing changes for docs.gptr.dev until you choose to switch.

What's included

• The current homepage (docs/src/pages/index.js), ported to a Mintlify custom-mode index.mdx with the same sections, copy and styling: hero with the code card and copyable install command, feature grid, how it works, the Jev benchmark callout, cited papers and the closing CTA. It supports light and dark mode and adds focus rings, hover arrows on the feature cards and an animated benchmark bar. The navbar uses light/dark wordmark logos to match the current one.
• All 61 pages from docs/sidebars.js in the same order and groups, under a Documentation tab.
• All 7 blog posts under a Blog tab, keeping their existing /blog/<slug> URLs, with author and date shown.
• Brand color, logo, favicon, GitHub/Discord/homepage links, and socials carried over from docusaurus.config.mjs.
• A /docs/:slug* redirect so existing links like /docs/gpt-researcher/gptr/config keep working after a domain switch.

Conversion notes

• Page titles come from the frontmatter title or the leading # H1, and sidebar_label becomes sidebarTitle.
• Relative and /docs/... links are rewritten to absolute Mintlify paths. Images move to mintlify/images/.
• HTML was adjusted for MDX: void tags are self-closed, stray </img>/</br> are removed, and HTML comments become MDX comments.
• Links to .py/.ipynb examples point to the files on GitHub because Mintlify doesn't render them.
• One link that was already broken upstream (mcp-server/claude-desktop-integration) now points to mcp-server/claude-integration.
• docs/proposals and the pydoc docs/reference pages weren't in the sidebar, so they aren't migrated.

Testing

• mint validate passes.
• mint broken-links finds no broken links.
• Every page returns 200 and renders in mint dev.

To go live, connect this repo at dashboard.mintlify.com with mintlify as the docs directory, then point docs.gptr.dev at it. mintlify/README.md has the details.